1. Field of the Invention
The present invention relates to an optical pickup device for optically recording and reproducing information and a lens used for optical pickup, and more particularly to a technology for providing a thinner optical pickup device which is shared, in recording and reproducing information, among several types of recording media each having different recording wavelengths and which comprises an achromatic lens for controlling the occurrence of an axial chromatic aberration (appearing more clearly with short wavelengths as a defocused convergent beam on an information recording surface of a disc) in an object lens resulting from wavelength variation triggered by instantaneous changes in optical outputs from a light source due to an operation switchover between recording and reproduction.
The entire documents of Japanese Patent Application No. 2008-307316 including its specification, drawings and Scope of Claims, filed on Dec. 2, 2008, are hereby incorporated by reference in this specification.
2. Description of the Related Art
These days, optical disc devices for optically recording and reproducing information are widespread among consumers; for example, an optical disc device for recording and reproducing various types of information such as video, image and audio using an optical disc such as CD (compact disc), DVD (digital versatile disc) or BD (blue-ray disc) as a recording medium. These optical disc devices available now in the market take various forms. Of the devices, a portable device, a device to be incorporated in a laptop computer and an in-vehicle device, for example, need to be smaller and thinner, and optical pickup devices to be used in these devices naturally need to be smaller and thinner.
Moreover, there has been a strong demand, in recent years, for an optical disc device capable of recording and reproducing information with respect to all of BD, DVD and CD which are three types of recording media each having different specifications. To meet the demand, it is necessary to provide light sources having three different wavelengths in order to record and reproduce information with respect to these three different recording media. In order to converge a light beam on each of the recoding media having three different optical conditions, it is further necessary to provide optical components, for example, lenses and prisms, adjusted to each of the different optical conditions. As a result, it becomes difficult to reduce the size of an optical pickup device to be used in such an optical disc device.
Each of the optical discs, BD, DVD and CD, is basically 12 cm in diameter. Optical disc devices used in portable-type or notebook-type personal computers or in-vehicle optical disc devices mostly use 12 cm-diameter optical discs for recording and reproduction. A disc-radial size of any device (device area) should be unexceptionally large enough to store a disc. Therefore, reducing the device area does not create any added value except for a portable game machine, for example, which uses a small-diameter disc specially designed. Although the disc-radial size and shape of the optical pickup device to be used in the optical disc device slightly vary among optical disc devices and among optical pickup devices, there are established dimensions and shapes generally accepted by the industry. The optical pickup devices supplied by manufactures are, in fact, expected to meet the industrial standard. To supply a value-added device in terms of its size under the circumstances described so far, “a device reduced in thickness” can be a great appealing point.
Until recently, standard notebook computers available in the market were provided with an optical disc device having the thickness of 12.7 mm. Today, thinner notebook computers comprising very thin optical disc devices having the thickness of at most 9.5 mm have been launched and are increasingly appreciated by consumers. These very thin optical disc devices are now faced with the market's request for their adaptability for three different discs, BD, DVD and CD in recording and reproducing information.
An optical pickup device used for recording undergoes an axial chromatic aberration due to wavelength variation in a light source triggered by a difference in output between recording and reproduction operations. When a reproducing operation switches to a recording operation or vice versa, an optical output from a light source instantaneously changes, and a wavelength then correspondingly instantaneously changes. The axial chromatic aberration produced by such an instantaneous wavelength change is presented as a defocused convergent beam on an information recording surface of the disc.
The optical disc device constantly carries out the focus control of an object lens position so that the convergent beam is focused on the information recording surface of the disc. The focus control, which is designed to follow possible surface run-out when the disc is rotated, is limited to a few-kHz band; in other words, the focus control has a limited response speed of the order from a few hundred microseconds to a few milliseconds. However, the optical output from the light source changes so quickly as a nano-second order in the recording-reproduction switchover, which cannot be followed by the focus control. Therefore, the defocus due to the axial chromatic aberration occurs in every recording-reproduction switchover, and the amount of defocus exceeding a system tolerance value results in the deterioration of a recording quality and control stability of the optical disc.
A possible approach for controlling the axial chromatic aberration is to provide a lens made of a material capable of controlling wavelength dispersion. A material capable of controlling wavelength dispersion, however, generally has a small refractive index and thereby has a large lens curvature as compared with a material having a high refractivity. As a result, it becomes difficult to produce a lens having a high numerical aperture NA such as an object lens from the material.
Axial chromatic aberration occurs in a lens of high power, and mainly in an object lens in the case of an optical pickup device. The axial chromatic aberration is more conspicuous as the lens power is higher, in other words, as the numerical aperture NA of the object lens is higher. Optical pickup devices for CD and DVD recording too undergo the axial chromatic aberration. However, the impact of the axial chromatic aberration in these devices, in which the numerical aperture NA of the object lens is not very high, is not as large as any problem is produced in practical use. Therefore, an achromatic lens is generally unnecessary for CD and DVD. On the other hand, an object lens for BD has such a high numerical aperture NA as at least 8.5, and the dispersion of the lens material used therein increases over shorter wavelengths, which makes it difficult to disregard the impact of the defocus due to the axial chromatic aberration. Therefore, optical pickup devices for BD recording conventionally use achromatic lenses.
The achromatic lens is conventionally provided near the object lens for BD (for example, see Disclosed Japanese Patent Document (2002-092926 of the Japanese Patent Applications Laid-Open) because it is easier to assemble the lenses with the centers thereof equally positioned when the lenses are closely disposed than distantly disposed. In the presence of any eccentricity between the positions of the object lens and the achromatic lens, a chromatic difference of magnification generated in the recording-reproduction switchover makes the convergent beam on the information recording surface of the disc instantaneously shift laterally, thereby making the convergent beam off a track center (off-track). The occurrence of the off-track may lead to the deterioration of a recording and reproduction quality and tracking control stability.
The tracking control is conventionally performed by transferring a movable element in which the object lens is housed in a track direction. In such a case, when the recording-reproduction switchover takes place during the shift of the object lens in the case where the achromatic lens is immovably secured and the object lens alone is moved, off-track happens due to the chromatic aberration of magnification as described earlier. It is not possible to follow the off-track caused by the chromatic aberration of magnification by using the tracking control since the off-track takes place at an extremely fast speed far beyond a tracking control band.
There are a variety of known structures of achromatic lenses, and one of the structures particularly well-known is such that a refractive lens having a positive power and a refractive lens having a negative power are bonded. According to this structure, in order to correct the chromatic aberration of the object lens, the chromatic aberration of the bonded lens is excessively corrected (color enhancing) so that the chromatic aberration of the object lens is offset.
However, such a bonded lens is expensive, thick and heavy and unsuitable as a lens to be housed in the movable element which makes very fast and minute movements. In recent years, a lens composed of a diffractive lens and a refractive lens integrally combined has been often used as the achromatic lens in an optical pickup device. The technology relating to the lens is recited, for example, in the Japanese disclosed patent document (2005-322281 of the Japanese Patent Applications Laid-Open; Patent No. 3297505). The refraction-diffraction integrated achromatic lens is less expensive than the bonded lens, and can be produced as a relatively light and thin lens.
Below is described a structure and an operation of an optical pickup device capable of recording and reproducing information with respect to BD, DVD and CD. FIG. 25 is an illustration of a structure of an optical pickup device based on conventional technology. According to the conventional technology, an achromatic lens 20 is provided between a second object lens 13 and a second mirror 12 immediately below the object lens 13. A light beam emitted from a blue-violet semiconductor laser 2 as a second light source, which is a light source to be used for BD, is reflected by a beam splitter 6 and converted by a collimate lens 7 into a substantially parallel light flux. The light flux then passes through a wave plate 8 and a first mirror 9 and is reflected by the second mirror 12. The light flux further passes through the achromatic lens 20 and is converged on a second recording medium (BD) 14 by the second object lens 13. The light beam reflected by the second recording medium 14 traces back the route described so far to finally reach the beam splitter 6. In the return route, the polarization state of the light beam has been changed from that of the outgoing route by an action of the wave plate 8. Therefore, a large portion of the light beam having reached the beam splitter 6 passes therethrough and further passes through a beam splitter 5 and then enters a detection lens 15. The light beam having entered the detection lens 15 enters a light detector 16 with astigmatism applied thereto. The light detector 16 detects various signals for focus, tracking, RF and the like from the incoming light beam.
A light beam emitted from a red semiconductor laser 1a as a first light source, which is a light source to be used for DVD, passes through a diffraction grating 3 and a wave plate 4 and is then reflected by the beam splitter 5. The reflected light passes through the beam splitter 6 and is then converted by the collimate lens 7 into a substantially parallel light flux. The light flux passes through the wave plate 8 and is then reflected by the first mirror 9 to be finally converged on a first recording medium (DVD) 11 by a first object lens 10. The light beam reflected by the first recording medium 11 traces back the route described so far to finally reach the beam splitter 5. In the return route, the polarization state of the light beam has been changed from that of the outgoing route by the action of the wave plate 8. Therefore, a portion of the light beam having reached the beam splitter 5 passes therethrough and then enters the detection lens 15. The light beam having entered the detection lens 15 then enters the light detector 16 with astigmatism applied thereto. The light detector 16 detects various signals for focus, tracking, RF and the like from the incoming light beam.
A light beam emitted from an infrared semiconductor laser 1b as another first light source, which is a light source to be used for CD, is irradiated on a CD recording medium according to an operation similar to that of the DVD optical system described earlier, and the reflected light thereof enters a light detector 1. The light detector 1 detects various signals for focus, tracking, RF and the like from the incoming light beam. As a supplementary explanation, the BD, DVD and CD are loaded in a common loading unit (turn table), and these optical discs are replaced with one another when used.
In the recited structure is used a two-wavelength laser comprising the red semiconductor laser 1a and the infrared semiconductor laser 1b provided in close juxtaposition in a housing. This specification omits the description of the structures of the wave plate 8, first mirror 9, and beam splitters 5 and 6 acting on an optical path to be branched and selected, and the structures of the detection lens 15 and the light detector 16 for detecting the various signals because none of them is a significant constitutive element of the present invention, and many different structures of these elements have so far been publicly known. In FIG. 25, the light sources, beam splitters and light detectors and the like are illustrated on the x-y plane, while a section surrounded by a broken line is illustrated on the y-z plane for convenience.
The optical disc device carries out a focus position control of the object lens because the beam always has to be focused on the information recording surface of the disc even if rotational surface run-out of the disc takes place due to its deflection or mass eccentricity. In the optical pickup device, therefore, it is necessary to ensure a range in the z-axis direction within which the object lens is movable.
In FIG. 25, a thickness direction of the optical pickup device is the z-axis direction, and a height of the BD optical system which occupies a thickness of the optical pickup device is H. The breakdown of the height H are: a working distance WD of the second object lens 13; a lens thickness t1, a thickness t2 of the achromatic lens 20; a required focus movable range d1 of the second object lens 13 for a downward movement; and a light flux diameter φ before the reflection on the second mirror 12. It is necessary in practice to allow for a clearance between the second object lens 13 and the achromatic lens 20 and a clearance between movable and immovable units in addition to the before-mentioned elements; however, a description hereinafter given is based on the following equation.H=WD+t1+t2+d1+φ  [1]
A key factor in reducing the thickness of the optical pickup device is how to reduce the height H to the minimum. It is obvious that the BD optical system provided with achromatic lenses 20, as compared with the DVD/CD optical system, has a disadvantage in the pursuit of thinning its optical pickup device. Therefore, in an optical pickup device for BD recording in which achromatic lenses 20 are indispensable, it is an effective design measure to reduce the light flux diameter φ. However, such a measure involves reduction of a diameter of the object lens for BD, consequently leading to the reduction in the working distance WD. The less working distance WD increases the risk of collision between the disc and the object lens. That is a demerit of this measure.
In an optical disc device, tracking control is conventionally performed to follow the disc eccentricity, wherein the object lens is shifted in the track direction (radial direction of optical disc). When the diameters of the light flux and the object lens are made smaller, the amount of the shift of the object lens with respect to the light flux diameter increases, which may lead to the deterioration of an optical performance at the time of the lens shift and performance of the tracking control.
As described so far, it is important to reduce the height H of the optical system while ensuring the light flux diameter φ as large as possible, in order to realize a very thin high-performance optical pickup device. In an optical pickup device to be used in a very thin optical disc device having the thickness of 9.5 mm (hereinafter, referred to as a very thin optical pickup device), for example, its height H is required to be at most about 5 mm. An anticipated eccentricity of a 12-cm optical disc device is desired to be about 0.2 mm. The light flux diameter φ needs to be at least about 2 mm, otherwise, the optical performance at the time of the lens shift and the tracking control performance are more evidently deteriorated.
Assuming that φ=2 mm and the numerical aperture NA of the second object lens 13 is 0.85, a focal distance of the second object lens 13 is φ/(2/NA)=2/(2/0.85)=1.18 mm. When an object lens having the focal distance of 1.18 mm and the numerical aperture NA of 0.85 is designed using a generally adopted glass material, the thickness t1 of the lens is approximately 1.6 mm, and the working distance WD thereof is approximately 0.3 mm. As a result of the subtraction of these values with H=5 mm set as a target, (d1+t2)=H−φ−t1−WD=5−2−1.6−0.3=1.1 mm. Thus, the value allocated to d1 and t2 is 1.1 mm or less. The required focus movable range d1 of the second object lens 13 for downward movement desirably has a value of approximately 1 mm. Assuming that d1=1.0 mm, 0.1 mm is finally left for t2. An achromatic lens 20 having the thickness of 0.1 mm is too thin a lens and is unrealistic. In fact, it is necessary to allow for inter-component clearances and a dimensional tolerance in assembling the components, which eventually leaves less than 0.1 mm for the thickness t2 of the achromatic lens 20. As thus far described, it is practically impossible in a very thin optical pickup device to provide an achromatic lens 20 immediately below the second object lens 13 while ensuring the light flux having a desired value.